It is well known to sinter a mass of polycrystalline particles, such as diamond or boron nitride, in the presence of a suitable solvent-catalyst by means of a HP/HT press to form a compact with good particle-to-particle bonding. Apparatus and techniques for forming such compacts are disclosed in U.S. Pat. Nos. 2,941,248-Hall, 3,141,746-DeLai, 3,743,489 and 3,767,371. While such compacts have good abrading and cutting characteristics, they have low transvers rupture strength and are not readily adapted to cutting operations due to the difficulty in securing them to a tool holder.
In order to mechanically strengthen the polycrystalline compacts and provide a convenient means of bonding or clamping to a tool holder to form a cutting tool, it has been proposed to bond the compact to a thick substrate of cemented carbide. U.S. Pat. No. 3,745,623-Wentorf et al teaches sintering of the particle mass in conjunction with tungsten carbide to produce a composite compact in which the particles are bonded directly to each other and to a cemented carbide substrate. Such composite compacts have been widely used in the cutting and drilling arts, since the cemented carbide substrate can be clamped or bonded to a suitable tool holder to provide a cutting edge for a cutting or drilling tool.
The composite compacts produced by the prior art techniques generally have utilized a solvent-catalyst sintering aid, such as cobalt, to accomplish particle-to-particle bonding in the HP/HT press. Such compacts have been limited to low-temperature applications, because, as recognized in U.S. Pat. No. 4,288,248-Bovenkerk et al, they degrade at temperatures above approximately 700.degree. C. The thermal degradation derives from the use of catalytic metals, such as cobalt or aluminum as the sintering aid for bonding the diamond or boron nitride crystals and results in accelerated wear or catastrophic failure of such compacts when employed in high-temperature applications, such as drilling rock formations having compressive strengths above 20,000 psi.
Difficulty has been experienced in utilizing the composite compacts produced by the prior art techniqes for drilling rock formations with even intermediate compressive strengths, i.e., 10,000 to 20,000 psi. In such applications it is generally necessary to braze the compact to a metal-bonded carbide pin which is received in a drill crown. Since the strength of the braze bond or joint is directly related to the liquidus of the braze filler metal employed, it is desireable to use the highest liquidus filler metals possible. However, because of the thermal degradation potential, it has been necessary to use braze filler metals with a liquidus below 700.degree. C. Even then temperatures approaching those at which the crystalline layer is degraded are required.
To avoid this problem, U.S. Pat. No. 4,225,322-Knemeyer has proposed a process for brazing a composite compact, such as made by the prior art techniques, to a pin or stud with a high liquidus braze filler metal by applying heat to the pin, to the filler metal and to the compact substrate while cooling the crystalline diamond or boron nitride table with a heat sink. This process allows production of cutting elements for rotary drill bits which utilize the capabilities of the crystalline composite compacts within the limits created by the construction of the compacts and the differential heating of the various components of the cutting elements. The use of cobalt as the solvent-catalyst in the prior art composite compacts imposes a limit on the operating temperatures due to thermal degradation. In addition, the thick cemented carbide substrate, which is approximately six times the thickness of the polycrystalline table, creates a very significant moment arm through which the working forces applied to the crystalline table are transmitted to the braze joint, thus substantially multiplying the effect of such forces on the joint. Furthermore, internal stresses are created within the composite compact due to the differential heating of the substrate and crystalline table. Also, the material of the pin is stressed by the high temperatures employed in the brazing process.